Japanese Patent Laid-Open Publications Nos. 6-340069 and 7-246706 each disclose an electrostatic inkjet head generally illustrated in FIG. 8. The inkjet head, generally indicated by reference numeral 70, includes three major parts; cover plate 72, channel plate 74 and substrate 76. These parts are assembled and fixed to each other. The channel plate 74 made from a silicon plate is formed with a number of grooves on an upper surface thereof by etching. The grooves are covered with the cover plate 72, so that defined are ink-channels 78 for receiving ink, nozzles 80 for ejecting ink in the form of droplets, reservoir 82 for the accommodation of supplemental ink, and ink-inlets 84 for connecting ink-channels 78 to the reservoir 82.
Thinned portions of the channel plate 74 underlying the ink-channels 78 are diaphragms 86, each of which bears a common electrode 88 on one surface thereof confronting to the substrate 76. For each diaphragm 86, the substrate 76 is formed with a recess 90 adjacent to the diaphragm 86. Also, a driving electrode 92 is mounted on a bottom of each recess 90 leaving a gap between the common and driving electrodes, 88 and 92, respectively.
In operation, when a voltage is applied between the common and driving electrodes, 88 and 92, an electric attraction force is generated between the electrodes, which biases the thinned diaphragm 86 to bend toward the substrate 76. The displacement of the diaphragm 86 increases a volume of the ink-channel 78, which draws supplemental ink from the reservoir 82 through corresponding ink-inlet 84. Then, when the voltage application is turned off, the displaced diaphragm 86 returns its original position shown in FIG. 8. This pressurizes the ink in the ink-channel 78, ejecting an ink-droplet 94 through the nozzle 80.
As shown in FIG. 9, the bent diaphragm 86 presents the maximum displacement at its central portion 87a. That is, the central portion 87a provides the minimum gap between the driving electrode 90 and the diaphragm 86 when the diaphragm is deformed.
Generally, the electrostatic attraction force to be generated between two opposing electrodes is inversely proportional to the second power of the distance between the electrodes. This means that the central portion 87a that defines the minimum gap between the electrodes is subject to the maximum electrostatic attraction force, which in turn accelerates the displacement of the central portion 87a. Therefore, the central portion 87a is subject to the maximum stress whenever the voltage is applied between the electrodes. This deteriorates a durability of the diaphragm 86 as well as the head 70.
In addition, in the electrostatic inkjet head, the electrostatic attraction force generated between the opposing electrodes varies with the voltage applied between the electrodes. Ideally, the size of the ink-droplet or amount of ink to be ejected can be controlled by changing the voltage applied. However, this requires a plurality of voltage drivers, for example, low-voltage, middle-voltage, and high-voltage drivers. Disadvantageously, a cost of the driver increases exponentially with the voltage that the driver can supply. This means that incorporating plural voltage drivers in the inkjet head results in a drastic increase in cost of the inkjet head. Alternatively, it appears to be possible to change the amount of ink to be ejected by changing an opposing area of the electrodes. Practically, however, it is not effective for the high-density inkjet head to change the area of the opposing electrodes for the purpose of increasing the amount of ink to be ejected.